Conventional radio-frequency identification (RFID) tags are used to identify objects, including people. RFID tags provide an alternative to bar codes for distinguishing and recording product for purchase. RFID tags can result in labor savings to manufacturers, distributors, and retailers. Annual estimated saving for a larger retailer using RFID tags could amount to billions of dollars.
The typical prior art RFID includes a microchip and an antenna. The antenna can be in the form of a tuned induction coil. The operation is fundamentally simple. Typically, the microchip stores a unique identification code that can be detected when the antenna of the tag couples inductively with an antenna of the reader. This coupling changes the impedance, hence the load at the receiving antenna. The load can be modulated according to the stored identification code, by switching the coil in and out.
Conventional RFID tags can be characterized according to the following basic attributes. An active RFID tag includes a power source to operate the microchip and to ‘broadcast’ the signal to the reader. Semi-passive tags use a battery to operate the microchip, but use an induced current to operate the transmitter. Because these types of tags are more costly to manufacture, they are typically used for high-cost objects that need to be identified at greater distances. For a passive tag, the reader induces a current in the tag by emitting electromagnetic radiation. These tags are relatively cheap, and are effective up to ranges of about 50 meters, depending on the power of the transmitted RF signal.
The tag can be read-only, or read-and-write. In the later type, information can be added to the tag over time using, e.g., an electrically erasable programmable read-only memory (EEPROM). For example, the tag can store when it was read, or how often it was read.
RFID tags can also be distinguished according to the frequency at which they operate. The operating frequencies need to consider RF spectrum assignments made by regulatory agencies such as the FCC in the United States. Low frequency tags are generally cheaper to make than high frequency devices, and use less power. Different applications may also prefer different frequencies. For example, low frequency tags are more suitable for applications with a high fluid content, e.g., items under water, humans, fruits, water based products. High frequency tags provide a higher data rate and range. Also, because high frequencies tend to be line-of-sight, they can be useful at fixed location with a narrow field-of-view, for example, assembly lines and doorways.
One problem encountered with RFID tags is collision.
Reader collision can happen when one reader interferes with the signal of another nearby reader. This can be a problem in warehousing where multiple users may want to identify stock at the same time. This can result in multiple readings of the same tag, which need to be resolved. In the prior art, time division multiplexing has been used to overcome this problem. However, this increases the complexity and cost of the system.
Tag collision occurs when many tags are co-located. This can result in multiple simultaneous readings of different tags, which need to be resolved. A number of techniques have been proposed to mitigate such collisions. Most of these require complex protocols that slow down the process.
Therefore, there is a need for RFID tags that can be selectively operated.